I'm building a fairly large AUV (robot submarine, essentially) and have encountered difficult problems moving from simple test models to building a real hull; most of which centers around sealing the hull piercings. In brief, for a small submarine, it's a difficult problem to seal rotating shaft egress points - the techniques used on boats and real submarines allow some leakage, which is managed by bilge pumps - but a small vessel doesn't have enough buoyancy margin to allow taking on water. For other reasons, I don't want to pressurize the hull to match ambient. My current designs use electric propulsion and regular stepper motors for the rudders/planes, and that just isn't going to work long-term.
Analysis of the tradeoffs involved leads me to believe that hydraulic actuators are the best solution to get power out of the pressure hull into the props and control surfaces, and I'm looking for resources for primarily low-cost, secondarily small-size hydraulics. The pressures involved are not high by normal hydraulics standards; a couple of hundred PSIG would be quite adequate. I'm looking for:
- pump
- at least 2 double-acting cylinders, ideally bore ~60mm, stroke
- at least 1 hydraulic motor
- solenoid valves for the above
- hopefully an off-the-shelf manifold where I can add a pressure sensor, rather than having to machine my own manifold
Cylinders and motor should be, preferably, stainless steel.
I would like these to be either DIRT cheap or totally standardized products, so that I can be assured of buying at least one set of backup parts a little ways down the line (probably Dec of next year). I hope I can get all the above for under $500, though I realize this probably isn't realistic.
The problem is not mainly one of size (I can resize the hull, and add batteries, within a reasonable limit). The problem is more one of standardized hose fittings and price :(
My other option is to go with waterproof DC motors, but these are so exotically expensive (at least, the ones I've found) that I want to avoid that route - besides, hydraulic pipes are easier to run through bulkheads than electric cables. But if someone knows of a reasonably priced source of both stepper (preferably 4-phase unipolar) and regular DC motors that are waterproof to either 5 or 45 meters [two different project phases] I'd be interested to hear about it. Freshwater operation, for the moment.
Any thoughts would be greatly appreciated.